chore: exclude lockfiles from GitHub diffs

See https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github

Author: edvald

.gitattributes

@@ -173,8 +173,8 @@ TODO              text
 .gitattributes    text
 .gitconfig        text
 .htaccess         text
-*.lock            text -diff
-package-lock.json text -diff
+*.lock            text -diff linguist-generated=true
+package-lock.json text -diff linguist-generated=true
 *.npmignore       text
 *.yaml            text
 *.yml             text